Numerous anti-ski brake systems for vehicles are already known, in which vehicle operating condition parameters are detected by transducers and processed in an evaluation circuit into control signals for influencing the braking pressure whenever predetermined operating conditions arise, particularly those having to do with the slip that exists between the wheel and the road surface.
Anti-skid brake systems of this kind do provide vehicles thus equipped with superior braking performance as compared with vehicles having conventional brakes; on the other hand, however, it is difficult to reconstruct the course of events leading up to an accident should vehicles equipped with anti-skid brake systems be involved in the accident, because such vehicles leave practically no skid marks. The reason for this is that the usual anti-skid brake systems establish an amount of slip between the wheel and the road surface which does not quite result in a relative speed between wheel and road surface such as to cause the rolling tire to leave a skid mark.
It is an object to provide a method in which data arising due to operation of an anti-skid system, particularly under emergency conditions, can be stored or reconstructed so that, in case of accident, a reconstruction of the operating conditions, speeds, etc. of the vehicle having the automatic anti-skid braking system can be reconstructed; and, preferably, to so arrange the storage system that another vehicle could be commanded to operate in similar manner, for example under testing conditions, so that similar accidents can be avoided at a future time.
Briefly, the anti-skid or anti-brake lock system provides an output signal when it is effective, that is, when it provides control signals to be brakes of the vehicle. When such control signals controlling the brakes of the vehicle are generated, the signals are stored, for example in an electronic memory which may be part of a removable storage element so that the stored data can be analyzed at a later time, either, in case of accident, by police or courts or, for purposes of improvement, under testing conditions. In accordance with a feature of the invention, the storage of braking data is carried out by releasing a visible marker, such as paint or the like, on the pavement when the control signals applied to the brakes by the anti-skid system reach a certain level, or were initiated at predetermined speeds, the marking on the pavement then being similar to skid marks which, due to the operation of the anti-skid system, would not be visible or would not occur on the pavement.
In a preferred embodiment of the invention, a storage of the operating condition parameters is performed only when an evaluation circuit of the anti-skid brake system responds and, at the same time, a predetermined limiting value for deceleration is exceeded. As a result, a particular advantage is that unnecessary markings on the road surface are avoided, since storing of operating condition parameters is not required every time braking occurs with an anti-skid brake system in effect. In a further embodiment of the invention, storage is put into effect only if either the brake light switch or a brake pressure transducer responds. Thus further assurance is provided that on the one hand, storage of the operating condition parameters or marking of the road surface is performed only when braking has been initiated, while on the other hand it is assured that these processes will take place even if either the brake light switch or the brake pressure transducer is defective.
Furthermore, in accordance with a further embodiment of the invention, marking of the road surface is performed under critical operating conditions in such a manner that markings whose length is constant in terms of time are applied to the road surface; thus the length of the markings makes it possible to conclude what the speed of the vehicle was at the time they were made.
Particularly good results are furthermore attained if the storage means for operating condition parameters is accommodated in a sealed plug-connectable module, because on the one hand unauthorized access to the stored operating condition parameters is thereby prevented, while on the other the storage means is easily removed from the vehicle for the purpose of reconstructing operating conditions; by putting this storage means into other vehicles of the same type, it is possible to obtain an exact reconstruction of the braking event.
One method according to the invention has the further advantage that skid marks are generated briefly in a specifically defined manner, making it possible to reconstruct both the course and the speed of the vehicle.
An advantageous further embodiment of the method according to the invention is distinguished by the prolongation, during a test phase, of the braking pressure maintenance phase conventionally established in anti-skid braking systems; the result is increased brake slippage, and in turn the generation of sufficient frictional energy on the engagement surface of the tire that a skid mark is made.
According to a further preferred form of embodiment of the invention, a test phase of constant duration is specified, so that a conclusion as to the speed at the onset of the test phase can be drawn on the basis of the length of the skid mark in the test phase. If the onset of the test phase coincides with the first braking cycle, then an indication as to the speed at the onset of braking is obtained.
According to a further embodiment of the invention, the test phase is initiated only after the speed drops below a predetermined test vehicle speed; the duration of the test phase is made dependent on the speed at the onset of braking. In this manner, the length of the skid mark can be converted by calculation into the speed at the onset of braking.
In a further embodiment of the invention, test phases and phases of a braking event with a normal slip threshold are arranged in sequence, as a result of which not only the speed at the onset of braking but also the vehicle deceleration can be ascertained from the length of the skid marks or the distance between them.
Increased safety is furthermore attained according to a further embodiment of the invention in that the switching threshold for test slip is set to be at least approximately inversely proportional to the vehicle speed.
Finally, a further embodiment of the invention provides that in order to increase safety, the test phases become effective only with respect to the front wheels, or to one of the front wheels, and that the test phase is suppressed in predetermined critical operating conditions, which are ascertained on the basis of transverse acceleration, steering wheel deflection angle or the wheel deceleration gradient.